Image frequency rejection system



March 30, 1937. H. A. WHEELER 2,075,633

IMAGE FREQUENCY REJECTION SYSTEM 7 Filed A ril s, 1935 f "Bl-C5175? 4- v 'V V 11 2a 25 i I i 15 E i SUPHMEERODY/lf i I g 22 -I l l i so 5 i 2/ L L--- ---'---l---- I INVENTOR HAROLD A.WHEELER- ATTORNEYS Patented Mar. 30, 1937 l p I 2,07 5,683

UNITED STATES PATENT orrlcs IMAGE FREQUENCY REJECTION SYSTEM Harold A. Wheeler, Great Neck, N. Y., asslgnor to Hazeltine Corporation, a corporation of Delaware Application April 5, 1933, Serial No. 664,479

14 Claims. (01.250-20) The present invention relates to the selection are generally used to selectively couple the an of a radio-frequency signal being-transmitted tenna to the first tubeof, said receiver. More on one carrier frequency and the simultaneous specifically, the inductance coil of the first of reduction of interference being transmitted on the two tuned circuits is inductively coupled to other frequencies not greatly different from the an antenna circuit primary coil and is also infrequency of the desired signal, ductively coupled to a'coupling coil included in This invention contemplates the provision of the second tuned circuit. An element of the two coupling paths between the antenna circuit invention in its preferred form resides in posiand the first tube of a radio receiver. The first tioning this latter mentioned coil relative to the coupling path comprises a. highly selective couother two coils so that when the first tuned circuit 1 'pling circuit and the second path comprises an is tuned to a certain signal frequency, an image aperiodic coupling. The couplings which are in frequency signal causes two equal and opposite phase opposition are so proportioned as to be voltages to be induced in the coupling coil at equal for currents of a frequency difiering by the image frequency, thus minimizing the reany desired amount from the desired signal fresultant voltage at the image frequency. This quency to which the selective coupling circuit is results in a great attenuation of the image tuned. frequency currents over and abovethat due to The invention is particularly applicable to a the inherent selectivity of the first tuned circuit. superheterodyne type of receiver; It is well The first two tuned circuitsmay be individually known that the superheterodyne receiver is sensheldedand the coupling coil may constitute a sitive to signals of two frequencies when set to very fewturnsof wire coupled to the first tuned receive any given frequency. The two frequencircuit. Alternatively the coupling coil may concies differ from each other by twice the amount stitute the entire inductance of the secondtuned of the off-set or intermediate frequency to which circuit. By means of the circuitsjust described the intermediate-frequency amplifier is permathe image frequency currents may be greatly nently tuned. attenuated without any detriment to the sensi- 25 It is a common practice to employ a signal tivity of the receiver to the desired signal. selecting system to amplify the desired fre- Having thus briefly described this invention, quency of the two carrier frequencies to which attention is invited to the accompanying draw-' the superheterodyne receiver responds, the deing in which: v sired frequency herein being called the signal Fig. 1 is a circuit diagram of a preselector frequency, and which signal selecting system 'atcircuit for providing "image frequency supprestenuates the undesired carrier frequency or dission in accordance with this invention; turbance occuring at the other frequency, herein Fig. 2 shows the circuit of Fig. 1 illustrating called the image frequency the preferred structural details of the induct- The relative. attenuation of the image freances employed. quency can be increasedby simultaneously tun- Referring now to Fig. 1, there is shown an ing several selective circuits to the signal freantenna or input circuit coupled to a superquency. However, if the station transmitting heterodyne receiver by a tuned preselector. The

40 it is diflicult to differentiate between the signal connection l2, primary coil l4 and damping reand image frequencies. a sistance l3. This antenna circuit is designed in- It is the object of the present invention to accordance with U. S. Patent No. 1,852,710 to overcome this disadvantage in a superheterodyne L. A. Hazeltine. V 'receiver and provide a circuit for suppressing The first tuned circuit comprises secondary 45 currents of the image frequency. d coil [5 and variable condenser l8 connected in This and further objects of this invention will a closed series circuit. The primary coil I 4 is become apparent from the following speciiicainductively coupled to the ground end iii of tion and claims taken in connection with the tuned coil [5. The second tuned circuit comaccompanying drawing. prises coil l9, variable condenser 20, and cou- 50 In accomplishing the objects of thisdnvention pling coil l1, connected in a closed series circuit. the undesired image frequency currents are The coupling coil I1 is coupled to tuned coil l5 attenuated by a special arrangement of two in a mannerwhich will be described in more coupled tuned circuits constituting the presedetail below. The second tuned circuit is conlector of a radio receiver in place of those which nected to the superheterodyne receiver 2|. The 55 at the image frequency comes in rather strong, antenna circuit comprises antenna II, ground 40 first and second tuned circuits may be tuned simultaneously by unicontrol means 22, which is preferably extended to also tune the receiver 2 t.

v In Fig. 2, which shows the same circuit as Fig. 1, corresponding elements are designated by the same reference characters. Thetwo figures are identical except that in Fig. 2 the two preselector' coil assemblies are shown in cross-section, thereby showing a preferred arrangement of the coils. Secondary coil I5 is a single-layer helical coil wound on a cylindrical form 28. Coil I4 is a high inductance multi-layer coil mounted co-axially in one end of form 28. Coupling coil I1, consisting of a relatively few turns is wound overa portion of coil l5, and is separated therefrom by insulating material 30. In this figure, the two tuned circuits are shown as enclosed by individual shields indicated by dotted lines.

The mutual inductances between coils will be 0 designated as follows:

M1 between coils l4 and I 5. M2 between coils I5 and I1, M3 between coils l 4 and 11. Referring to Fig. 1, the operation at the signal frequency may be explained as follows. The two tuned circuits are tuned to the signal of the desired frequency by means of condensers l8 and 20. The receiver 2| is also tuned to the signal. The signal currents are transferred by M1 from antenna circuit to first tuned circuit, where they are built up by resonance to a much greater value. M2, and a second resonant current is built up in the second tuned circuit. The resulting voltage across condenser 20 is coupled to the receiver 2|. The voltage induced in coil I'I directly through M3 is negligible because the antenna current in coil I4 is so much smaller than the resonant current in coil l5, at the signal fre- 40 quency.

Having tuned the system of Fig. 1 to the signal frequency, the superheterodyne receiver is also incidentally sensitive at the image frequency which is herein assumed to be higher than the signal frequency and which diifers therefrom by twice the intermediate frequency. The behavior of the preselector toward image frequency signals may be explained as follows.

The antenna current in coil 14 induces a voltage,

in coil l5 due to M1 and in coil I I due to' Ms. A current is thereby caused to fiow in the first tuned circuit, which induces another voltage in coil l1 clue to M2. The first tuned circuit is not resonant at the image frequency and therefore the image frequency current incidentally induced therein is comparable with the antenna current. By adjusting any one or more of the values M1, M2 and M3, a combination is found which makes equal and opposite the two voltages which are 0 induced in coil l1. As a result, the image frequency signal is not transferred to the second tuned circuit and to the receiver 2|, but is substantially completely suppressed. I

The condition for complete image suppres- 5 sion is given by the equation:

MlM g MaL :2

in which L is the inductance of coil I5, is is the v A voltage isinduced in coil I! through fore on the order of 0.5, being always less than unity and greater than zero.

The above equation indicates the desired condition that the product M1M2 should be on the order of one-half the product MzL, and should always be less than MsL. This relationmay be satisfied by a number of coil arrangements in space. Fig. 2 shows a preferred arrangement,

and has been described in a general way, with reference to Fig. 1. The above equation is easily satisfied by the following conditions and experimental procedure. placed in the same direction from the center of coil l5, and coil I 4 is displaced farther than coil II. In other words, the center of coil I1 is located between the centersof coils l4 and IS. The value of M1 required for effective signal transfer is first obtained, by the correct distance between, centers of coils l4 and I5. Leaving these coils fixed, coil I1 is moved along the surface of coil I 5 until a position is found where the image frequency is balanced out. In this procedure, M1 and L are unchanged, while M: andMs vary in opposite directions, there being only one position of coil I I which satisfies the above equation and results in balancing out the "image frequency. In an experiment using a 1000 kilocycles signal and 125 kilocycle intermediate frequency, the correct position of the coil.

I! was found to be about one-eighth of the way along secondary coil 15, from the ground end It.

In a. given receiver, f1 and f2 vary over awide range but their difference is constant. equal to twice the intermediate frequency. Therefore the ratio fl /fil varies somewhat with tuning. Using the specific circuits of Figs. 1 and 2, the same positioning of coils l4, l5 and VI! will not'exactly tenna, preselector, and receiver.

Whereas this invention, as described, is applicable to the preselector circuit of a superheterodyne radio receiver, it is to be understood that the principles involved are equally applicable 'to any selector network for use in connection with any type of receiver orto interstage coupling circuits for use generally with any radio responsive devices.

I claim:

1. In a superheterodyne radio receiver, means for attenuating image frequency currents, which comprise a single signal input circuit, a circuit tuned to the desired frequency and coupled to said input circuit for building up by resonance Coils and I! are both discurrent of the signal frequency impressed on said input circuit and incidentally transferring current of the image frequency, and a coupling coil coupled to said tuned circuit and'aperiodically coupled to said input circuit, said couplings being proportioned to induce in said coil equal and opposite voltages in response to image frequencycurrents in said input circuit.

2. In a superheterodyneradio receiver, means for attenuating image frequency currents, which comprise a single signal input circuit, a circuit tuned to the desired frequency and inductively coupled to said .input circuit for building up by resonance current of the signal frequency impressed on said input circuit and incidentally transferring current of the image frequency, and a coupling coil inductively coupled to said tuned circuit and aperiodically coupled to said'input circuit, said couplings being proportioned to induce in said coil equal and opposite voltages in response to image frequency currents in said input circuit.

3. 'A selector network for selecting currents of frequency f1 and attenuating currents of frequencyfz, comprising a primary coil, a secondary coil tuned to f1 and having inductance L, and a coupling coil; said coils having mutual inductance M1 between primary and secondary, mutual inductance M2 between secondary and coupling coil, and mutual inductance Ma between primary and coupling coil; the voltages induced in the coupling coil by currents of 12 in the primary and secondary coils being at least partially balanced out by positioning the coils to make M1M2 on the order of one-half of MaL.

4. A selector network for selecting currents of frequency f1 and rejecting currents of frequency Jz, comprising a primary coil, a secondary coil tuned to f1 and having inductance L, and a coupling coil; said coils having mutual inductance M1 between primary and secondary, mutual inductance M2 between secondary and coupling coil, mutual inductance M3 between primary and coupling coil, and being positioned so that the equation M M {g M L is satisfied, whereby the voltages induced in the coupling coil by currents at f2 in the primary and secondary coils are balanced out in the coupling coil.

5. A selector network for selecting currents of a desired frequency and attenuating currents of an undesired frequency, comprising a single signal input circuit, a first circuit tuned to the desired frequency for building up by resonance current of the signal frequency impressed on said input circuit and incidentally transferring current of the undesired frequency, and a second circuit tuned to the desired frequency for further building up by resonance current of the signal frequency, said first tuned circuit being coupled to said input circuit and said second tuned circuit being coupled to said first tuned circuit and aperiodically coupled to said input circuit, said couplings being so proportioned that currents of the undesired frequency in the input circuit cause two opposing voltages of such undesired frequency to be induced in said second tuned circuit.

6. A selector network for selecting currents of a desired frequency and attenuating currents of an undesired frequency, comprising a single signal input circuit, a first tuned circuit tuned to the desired signal frequency for building up by resonance current of the desired frequency impressed'on said input circuit and incidentally transferring current of the undesired frequency,

-' and a second tuned circuit tuned to the signal frequency for further building up by resonance current of the desired frequency, said first tuned circuit being inductively coupled to each of the other circuits and said second tuned circuit being aperiodically coupled to said input circuit,

said couplings being so proportioned that currents of the undesired frequency in the input circuit cause two equal and opposite voltages of such undesired frequency to be induced in the second tuned circuit.

7. A. preselector circuit for transferring desired currents, but not image frequency currents, from an antenna to a superheterodyne receiver, comprising a primary coil connected to said antenna, a helical secondary coil tuned to coupled to the primary coil, and a coupling coil the desired frequency and having one end coupled to said receiverysaid coupling coil having relatively few turns wound over the secondary coil in a position intermediate the ends thereof, said position being chosen to minimize the voltage induced in the coupling coil at the image frequency.

8. A preselector circuit for transferring desired currents, but not image frequency currents, from an antenna to a superheterodyne receiver, comprising a primary coil connected to said antenna, ahelical secondary coilv tuned to the desired frequency and having one end coupled to the primary coil, and a coupling coil coupled to said receiver, said coupling coil having relatively few turns wound over the secondary coil ina position about one-eighth the way along the secondary coil from said end.

9. In a superheterodyne radio receiver, means for attenuating image frequency currents, which comprise a single antenna circuit including an inductance; a first tunable circuit for building up by resonance current of the signal frequencyimpressed on said antenna circuit and incidentally transferring current of the, image frequency, said circuit including a variable condenser and an inductance inductively coupled to said antenna inductance; a second tunable circuit including a variable condenser and an inductance coupled to said antenna inductance and aperiodically coupled to the inductance of said first tunable circuit; and a superheterodyne receiver coupled to-said second tunable circuit, the mutual inductances of said circuits being so chosen that equal and opposite currents are transferred to the second tunable circuit from the first tunable circuit and the antenna circuit, respectively, at the image frequency.

10. In a superheterodyne radio receiving system, means for attenuating image frequency currents. which comprise a single antenna circuit including an inductance; a first tunable circuit for building up by resonance current of the desired frequency impressed on said antenna circuit and incidentally transferring current of the image frequency and comprising an inductance and a variable condenser, said last-mentioned inductance being inductively coupled to the inductance of said antenna circuit; a second tunable circuit comprising an inductance, a coupling coil inductively coupled to the inductance of said first tunable circuit and aperiodically coupled to the inductance of said antenna circuit, and a variable condenser; and a superheterodyne receiver connected in shunt across said last-mentioned variable condenser, the mutual inductances of "said circuits being so chosen that equal and opposite currents are transferred to the sec-- cuit and the antenna circuit, respectively, at the image frequency when both of said tunable circuits of' said receiver are tuned to receive currents of a desired signal frequency.

11. In a superheterodyne radio receiving system, means for attenuating image frequency cur- ,rents, which comprise a single antenna circuit including a high inductance; a first tunable circuit for building up by resonance current of the desired frequency impressed on said antenna circuit and incidentally transferring current of the image frequency and comprising an inductance and a variable condenser, said last-mentioned inductance being inductively coupled to the in- 5 ductance of said antenna circuit, a second tunable circuit comprisingan inductance, a coupling coil inductively coupled to the inductance of said first tunable circuit and aperiodically coupled to the inductance of said antenna circuit,' and a variable condenser; and a superheterodyne receiver connected in shunt across the last-men- 'tioned variable condenser, both of said variable condensers being connected for uni-control operation together with the tuning elements of said 5 receiver, the mutual inductances of said circuits being so chosen that equal and opposite currents are transferred to the second tunable circuit from the first tunable circuit and the antenna circuit, respectively, at the image frequency when both 20 of said tunable circuits of the receiver are tuned to receive currents of a desired signal frequency. 12; In a superheterodyne radio receiver, means for attenuating imagev frequency currents, which comprise a single antenna circuit, a first tunable circuit coupled to said antenna circuit for building up by resonance current of the desired signal frequency impressed on said antenna circuit and incidentally transferring current of the image frequency, and a second tunable circuit aperiodi- 30 cally coupled to said antenna circuit and coupled to said first tunable circuit, said couplings being so proportioned that when said tunable circuits said input circuit and effective incidentally to induce in said coila voltage of the image frequency, and an aperiodic coupling path between said circuit and said coil for inducing in said coil 9. voltage of the image frequency equal and opposite to said first-mentioned image-frequency voltage.

14. In a superheterodyne radio receiver, means for attenuating image-frequency currents comprising a single signal input circuit, a coupling coil, a path between said circuit and said coil including means for building up by resonance current of a desired signal frequency impressed in said input circuit and effective incidentally to induce in said coil a voltage of the image frequency, and a fixed aperiodic coupling path between said circuit and said coil for inducing in I said coil a voltage of the image frequency equal and opposite to said first-mentioned image-frequency voltage.

HAROLD A. WHEELER. 

